1. Field of the Invention
The subject invention relates to vibration transducers having a seismic mass damped with a viscous medium.
2. Description of the Prior Art
Many types of prior-art transducers employ bearings for mounting a seismic mass with respect to a relatively stationary transducer part. In practice, the friction of such bearings causes distortion of the transducer signal. Also, the use of bearings introduces a minimum drive level to overcome static friction before the transducer starts to perform properly. Moreover, the inevitable wear of bearings shortens transducer life.
A more advantageous prior-art vibration transducer avoids bearing friction by suspending the seismic mass between spring systems which are flexible in an axial direction of the mass but have their maximum stiffness at right angles to such axial direction. Typically, the spring systems are interconnected by a bridge which extends past the seismic mass. This assembly is enclosed in a housing.
In most practical applications, the air in the housing would not be sufficient for a satisfactory damping of the seismic mass. Rather, the seismic mass has to be immersed in a viscous liquid, typically oil, for providing the requisite damping characteristics.
In many applications the latter type of transducers is satisfactory and, indeed, has been manufactured and sold for many years by Bell & Howell Company as Vibration Transducers Types 4-102-0001 and 4-103-0001, for instance.
However, the presence of a viscous liquid, typically oil, as a damping medium in the transducer has become an impediment in a growing number of applications. For instance, positive hermetic sealing is required to prevent loss of the damping liquid over the entire operating range. The transducer has to be equipped with a special expansion cell to compensate for the thermal expansion of the damping liquid when temperature rises. Even with these precautions and additional features, the useful temperature range of the transducer is inevitably limited both in the direction of relatively low temperatures and in the direction of high temperatures. In fact, use of a damping oil tends to restrict the useful temperature operating range of a vibration transducer to temperature spans which are even narrower than the span between water freezing and boiling temperatures.
On the other hand, use of a damping medium, such as air or another gas, which would not subject the transducer to the above mentioned disadvantages of damping oils, would on the basis of existing transducer technology necessitate a return to bearings for impeding the flow of the damping medium past the seismic mass.
This, of cours, would reintroduce the above mentioned serious disadvantages of bearing devices in vibration transducers.